Total Solar Irradiance (TSI) Monitoring

Figure 1 (Click for a high resolution pdf version)  

The Earth’s weather and climate regime is determined by the total solar irradiance (TSI) and its interactions with the Earth’s atmosphere, oceans and landmasses. Evidence from both 29 years of direct satellite monitoring and historical proxy data leaves no doubt that solar luminosity in general, and TSI in particular, are intrinsically variable phenomena. Subtle variations of TSI resulting from periodic changes in the Earth's orbit (Milankovich cycles: ~20, 40 and 100 Kyrs) cause climate change ranging from major ice ages to the present inter-glacial, clearly demonstrating the dominance of TSI in climate change on long timescales. TSI monitoring, cosmogenic isotope analyses and correlative climate data indicate that variations of the TSI have been a significant climate forcing during the current inter-glacial period (the last ~ 10 Kyrs.). Phenomenological analyses of TSI monitoring results during the past (nearly) three decades, TSI proxies during the past 400 years and the records of surface temperature show that TSI variation has been the dominant forcing for climate change during the industrial era. The periodic character of the TSI record indicates that solar forcing of climate change will likely be the dominant variable contributor to climate change in the future.

A series of Active Cavity Radiometers (ACRs), a new generation of sensors with the precision required for compiling a long term TSI database for climate, have been developed at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology. Their use in a series of Active Cavity Radiometer Irradiance Monitor (ACRIM) space flight experiments has provided a precise and traceable component of the TSI database during nearly 90 % of its 29 year history. The ACRIM Science Team,  directed by the ACRIM Principal Investigator, Dr. Richard C. Willson, moved the Science Team to Columbia University in 1995. The ACRIM Instrument Team, directed by ACRIMSAT Program Manager Roger Helizon, continues to operate the ACRIM flight  and ground telemetry instrumentation at JPL.

A contiguous TSI database of satellite observations extends from late 1978 to the present, covering more than two sunspot cycles. It's comprised of the observations of seven independent experiments: Nimbus7/ERB¹, SMM/ACRIM1², ERBS/ERBE³, UARS/ACRIM2⁴, SOHO/VIRGO⁵, ACRIMSAT/ACRIM3⁶ and SORCE/TIM⁷. A composite database combining these results using overlapping, in-flight comparisons has begun to provide new insights into both solar physics and climate change.

Monitoring TSI variability is clearly an important component of climate change research, particularly in the context of understanding the relative forcings of natural and anthropogenic processes. The requirements for a long-term, climate TSI database can be inferred from a recent National Research Council study which concluded that gradual variations in solar luminosity of as little as 0.25 % was the likely  forcing for the ‘little ice age’ that persisted in varying degree from the late 14^th to the mid 19^th centuries. A centuries-long TSI database will have to be calibrated by either precision or accuracy to a small fraction of this value to be of any use in assessing the magnitude of solar forcing. The current TSI database is shown in Figure 1.

ACRIM TSI Resource Center

Figure 2 (Click for a high resolution pdf version)

Instrumentation used in spaceflight TSI monitoring to date has utilized sensors operating at ambient temperature (~ 20C). They provide the only practicable satellite instrument technology presently available for extended flight experiments. The ‘native scale’, on which the results of each experiment’s TSI observations are reported, is based on the metrology of their individual cavity sensor properties in the International System of units (SI).

The de-facto redundant, overlap TSI monitoring approach that has provided a contiguous record since 1978 resulted from the deployment of multiple, overlapping TSI satellite experiments. The traceability of this database is at the mutual precision level of overlapping experiments. This is typically orders of magnitude smaller than the ‘absolute uncertainty’ of observations in SI units. ACRIM3 results have demonstrated a residual annual traceability of ~ 5 ppm during its 8 year mission. A carefully implemented redundant, overlap strategy should therefore be capable of producing a centennial TSI record with traceability of ~ 500 ppm, providing a useful signal to noise ratio for assessing climate response to TSI variation.

• A redundant, overlapping TSI measurement strategy using existing ‘ambient temperature’ instrumentation can provide the long term traceability required by a TSI database for climate change on centennial time scales.

The state of the art measurement uncertainty for flight observations on an ‘absolute scale’ in the international system of units (SI) has not been demonstrated to be significantly less than 1000 parts per million (ppm). The results of TSI monitoring experiments are reported on their 'native scales' as defined in SI by the ‘self-calibration’ features of their sensor technologies. Systematic uncertainties in the metrology used to relate their observations to SI caused the ± 0.25 % spread of results during the first decade of monitoring. The tighter clustering of results after 1990 is attributable to dissemination of more accurate sensor metrology among the various experiments and national standards labs. The causes of the ~ -0.35 % difference between the ACRIM3 and VIRGO results and the SORCE/TIM results are not presently understood in this context.⁸

• A TSI monitoring strategy that relies on ‘absolute uncertainty’ for its long term traceability cannot provide the long term traceability required by a TSI database for climate change on centennial time scales.

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1.  Nimbus7/Earth Radiation Budget experiment (1978 - 1993)

2. Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor 1 (1980 - 1989)

3. Earth Radiation Budget Satellite/Earth Radiation Budget Experiment (1984 - 1999)

4. Upper Atmosphere Research Satellite/Active cavity Radiometer Irradiance Monitor 2 (1991 - 2001)

5. Solar and Heliospheric Observer/Variability of solar Irradiance and Gravity Oscillations (1996 --->)

6. ACRIM Satellite/Active cavity Radiometer Irradiance Monitor 3 (2000 --->)

7. Solar Radiation and Climate Experiment/Total Irradiance Monitor (2003 --->)

8. Despite significant effort on the part of the National Institute of Standards and Technology during recent years.